Absorbent fibers useful as components in advanced wound care dressings are known in the art, particularly fibers based on alginic acid, carboxymethylcellulose, and carboxymethylchitosan, and salts thereof.
Dressings based on fibers of alginic acid or its salts have good overall absorbency of wound fluid, but suffer from slow absorption due to the need to exchange multivalent ions binding the fibrous structure together with sodium ions present in wound fluid. Although this ion exchange renders the fibers swellable in ion-containing aqueous media, allowing significant absorption of fluid, the mechanical strength of the gelled fibers is compromised, and it is not routinely possible to remove a saturated dressing in one piece. Frequently, the dressing must be irrigated with saline to wash it away, and this can be traumatic for the patient.
Carboxymethyl cellulose fibers have also been used as the main component in advanced wound care dressings, and these too have significant absorptive capacity for wound fluid. Their advantage over alginate-type dressings is that absorption of fluid is virtually instantaneous since no ionic exchange is required to render the fibers gellable. In addition, those fibers based on a highly crystalline cellulose, such as lyocell, and in particular those described in EP0616650 and EP0680344, tend to retain a higher level of mechanical strength and therefore may be removed from the wound site in one piece. However the absorptive capacity of this class of material is strongly dependent on the pH of the wound fluid, reducing dramatically at acidic pH. This is a serious drawback since chronic wound fluid pH can range between 4 and 8 depending on the state of healing. Furthermore, it has been recognized that artificially lowering the pH of the wound environment may lead to improved healing outcomes. For instance, it has been found (Tsioras et al, article presented at 19th Annual Symposium on Advanced Wound Care, San Antonio, Tex., Apr. 30, 2006-May 3, 2006) that applying a wound dressing containing a pH adjusting cream of pH 2.8 decreased the time it took for the wound to close. In another study, burn wounds healed quicker when treated with fluid having a pH of 3.5 (Kaufman et al., Burns Incl Therm Inj, 12(2) 84-90 (1985)). Indeed, preparations are commercially available for use in conjunction with absorbent dressings to reduce the pH of the wound environment. For instance, CADESORB® available from Smith & Nephew has a pH of about 4.35.
It is desirable for an absorbent dressing to perform well at acidic pH, and preferably for it to perform well over a wide range of pH. Since absorbent dressings based on carboxymethylcellulose do not perform well in low pH environments, there is a need for an instantly gelling, absorptive dressing that continues to absorb to a good level at reduced pH.
It is desirable for absorbent fibers for use in absorbent dressings to be obtained from a renewable resource, to be inexpensive and also biodegradable. Hence, there is considerable interest in cellulose as a renewable and biodegradable source of absorbent material. U.S. southern pine fluff pulp is used as an absorbent material in the personal care industry. However, it is commonly used in conjunction with other absorbent materials, and commonly materials that are not renewable and biodegradable, for example acrylic acid polymers. The reason for this is that absorbed liquid is not effectively retained in materials that are made exclusively of cellulosic fibers.
The cellulose fiber can be modified by sulfonation, for example by substitution with an alkyl sulfonate at one or more of the hydroxyl groups on the anhydroglucose monomers that make up the cellulose backbone, forming ether linkages. Cellulose derivatives of this type are known as cellulose sulfonates or cellulose alkyl sulfonates.
Commercially available cellulose ethers are, as a rule, water-soluble compounds. In particular, cellulose ethyl sulfonate is known to be water-soluble.
Herzog et al., U.S. Pat. No. 4,990,609 describes cellulose ethyl sulfonates of high solution quality, which are prepared by addition to cellulose of an alkylating agent and subsequently addition of alkali. The process is compared to the two-stage process for the production of cellulose ethyl sulfonate described in SU757540.
Cellulose ether sulfonates have been modified further in order to produce water insoluble products. For instance Glasser et al., U.S. Published Patent Application No. 2006/0142560 refers to absorbent fibers based on mixed cellulose alkylsulfonates in which the cellulose is substituted by two different groups, an alkyl sulfonate and a hydroxyalkyl sulfonate, specifically ethyl sulfonate and 2-hydroxypropyl sulfonate. Water insolubility of the modified cellulose is believed to result from the presence of the 2-hydroxypropyl sulfonate group.
Shet et al., U.S. Pat. No. 5,703,225 refers to a water-insoluble sulfonated cellulose that is a hydroxy sulfonic cellulose in which both the sulfur atom of a sulfonic group and a hydroxyl group are directly attached to a carbon atom on the cellulose chain.
To be suitable for use in wound dressings, absorbent materials must retain their integrity and hence be water-insoluble. The principal disadvantage of the water insoluble cellulose alkyl sulfonates that have been developed for use as absorbent materials to date is the requirement for substitution of the cellulose with at least two different groups. Compared to substitution with a single substituent, additional reactants and additional processing steps are not desirable, and are likely to increase the cost of manufacture. Furthermore, as the cellulose is increasingly modified, benefits associated with the natural fiber, such as its biodegradability, may be impaired.